Nitrogen oxides, typically called “NOx”, have been implicated in the formation of ground level ozone and acid rain. Combustion of pulverized coal, or other nitrogen bearing hydrocarbonaceous fuels, in boilers represents a significant source of NOx emissions. Current and pending regulations require that NOx emissions from these facilities be reduced significantly from present values.
One technology that is currently used to reduce NOx emissions is air staging in the furnace. In this technology the airflow to the burners, which may be low NOx burners, is typically less than that required to completely combust the fuel. This arrangement creates a fuel-rich zone that tends to drive the nitrogen from the fuel to react to form molecular nitrogen instead of NOx. The remainder of the combustion air is introduced through air ports above the burner zone. The air introduced in this manner is termed “overfire air” herein and in this field. Although effective, this arrangement can be problematic for some boilers.
For example, boilers in which the residence time between the top row of burners and the overfire air is short and/or the residence time between the overfire air ports and the furnace exit plane is short. Under these conditions there is not enough residence time available to completely combust the CO formed in the fuel rich stage. When coupled with a high heat release rate, defined as the heat release per unit furnace volume in the burner zone, this can also lead to significant amounts of combustion taking place in the upper furnace. This combustion in the upper furnace, in turn, can cause problems with the steam temperature control and can cause additional NOx formation—in some cases actually negating any benefit of staging in the burner zone.
The use of air-staging for NOx control is well known and is based on a competition between reactions that convert fuel nitrogen to N2 and oxidation of fuel nitrogen to NO. The net result of these competing reactions can be shown in FIG. 1, which represents a typical staging curve. When the burner is operated under fuel lean conditions the flame temperature is high and extra oxygen is available. Under these conditions the oxidation reactions dominate which leads to increased NOx formation, particularly when coupled to the increase in thermal NOx formation under these conditions. Under very fuel rich conditions the flame temperatures are reduced since less heat is released. Consequently when fuel nitrogen is released from the coal there is very little conversion of these species to either N2 or NOx in the fuel rich zone. When the overfire air is added the fuel nitrogen species are oxidized to NO.
Many investigators, such as Bowman, C., “Control of Combustion-Generated Nitrogen Oxide Emissions: Technology Driven by Regulation”, Twenty-Fourth Symposium (International) on Combustion, The Combustion Institute, 1992, and Johnson, S., Yang, R., Sommer, T., “Interpretation of Small and Intermediate Scale Test Results from a Low NOx Combustion System for Pulverized Coal”, International Flame Research Foundation Meeting, Noordwijkerhout, Holland, May 1980, suggest that the optimal stoichiometric ratio for NOx emissions is in the range of approximately 0.65-0.80. In general careful attention is paid to achieve overall stoichiometric ratios near this optimum. Based on this staging curve the concept of operating some burners extremely fuel rich and others extremely fuel lean would seem to yield the worst possible conditions for NOx control—not the observed reductions in NOx formation from this invention.
In conjunction with the concept of tuning the burner stoichiometric ratio to a specific value U.S. Pat. No. 6,699,030 teaches that oxygen injection can be used to minimize variations in stoichiometric ratio from burner to burner. In one example this patent teaches to increase the flame temperature of those burners operating at the bottom of the boiler or near the walls. In contrast, the present invention described herein takes advantage of the naturally occurring lower flame temperatures found in these regions coupled with dramatically different burner stoichiometric ratios to achieve low NOx.
Other investigators have disclosed methods of reducing NOx emissions by increasing the temperature in the fuel rich zone. For example, Eddings, E., Sarofim, A., Adams, B., Harding, S., Heap, M., “Advances in the Use of Computer Simulations for Evaluating Combustion Alternatives”, The 3rd CREST International Symposium on High Temperature Air Combustion and Gasification, Yokohama, Japan, Mar. 6-9, 2000, discusses the potential impact of high air preheat temperatures on NOx emissions under fuel rich conditions. However, although increasing the air preheat temperature would lead to lower emissions under fuel rich conditions it could actually exacerbate some of the problems discussed above with staging in constrained units, particularly steam temperature problems. High air preheat temperatures would also increase the NOx formation under lean conditions unlike the current invention. Kobayashi, H., Bool, L. E., “Oxygen Enhanced Low NOx Combustion”, U.S. Patent Application No. 20030009932, published Jan. 16, 2003 and Kobayashi, H., Bool, L. E., , “Oxygen Enhanced Low NOx Combustion”, U.S. Patent Application No. 20020127505, published Sep. 19, 2002, disclose methods of increasing the local flame temperatures by replacing a portion of the combustion air with oxygen. Again, conventional wisdom would suggest that adding oxygen under lean conditions would cause an increase in the net NOx emissions. However the current invention was shown to actually reduce NOx emissions when oxygen was added to the fuel lean burners.
The concept of staggered burner stoichiometric ratios for NOx control is discussed by U.S. Pat. No. 5,387,100, This patent discloses a method of alternating fuel rich and fuel lean burners such that the exhaust gases of both mix downstream in the furnace to complete combustion. By using both very fuel rich and very fuel lean burners the goal is to minimize the flame temperature in all the burners, and therefore minimize the thermal NOx formation. In the present invention the goal is to achieve high flame temperatures in the fuel rich burners—the opposite of that taught in this patent.